The subject matter of the present specification relates to controllers to operate discharge lamps, e.g., fluorescent lamps.
In general, a fluorescent lamp assembly is made from a tube filled with an inert gas, for example, argon, and some low pressure mercury vapors. The inside of the tube is coated with a fluorescent material. The fluorescent lamp includes two electrodes attached to either end of the tube. The electrode includes a filament surrounded by an emitting cathode. When the filament is warmed up, the cathode emits thermal electrons that form an electron cloud around the cathode. When a potential difference is applied between the filaments, the charge carriers accelerate towards the positive electrode. While migrating towards the positive electrode, the charge carriers collide with one or more mercury atoms. If the energy with which a charge carrier collides with a mercury atom is sufficiently high, then the mercury atom emits ultraviolet radiation. When the coating on the inside of the tube receives the ultraviolet radiation, the coating emits radiation in the visible spectrum, which appears as light.
Electronic ballasts are typically used to operate fluorescent lamps, such operations including switching the lamps on and off, dimming the lamps, and the like. An electronic ballast for fluorescent lamp dimming control can use several circuits, for example, a series LC resonant series loaded circuit, a series resonance parallel loaded circuit, a series parallel resonance circuit, and the like. The series circuits are typically controlled by either frequency or the duty cycle of symmetrically chopped input voltage pulses. A series resonance parallel loaded circuit, used in an electronic ballast, behaves like a low-pass filter and shows a high gain at high impedance that is required by the fluorescent lamp during ignition and low dimming. The input of the electronic ballast comes from a source, for example, a DC source. Two switching elements turn on and off in response to a signal from a controller to convert the DC voltage into an AC voltage. The controller controls states of the switching elements, and thus the waveform of the AC voltage, in accordance with a desired dimming level. By controlling the timing of turning the switching elements on and off, the current flowing through a fluorescent lamp can be changed, and the light output of the fluorescent lamp can be varied. An electronic ballast operated using a series resonance parallel loaded circuit that includes double switch choppers at the DC output is generally adjustable, providing the high voltage required for lamp ignition, short circuit proof, and offers increased voltage in high impedance and low load during dimming.